In the manufacture of disc drives, servo information is written onto discs to define tracks along which the read/write heads follow in drive operations. Despite the fact that servo information is written using a high precision servo track writer, the tracks rarely, if ever, turn out to be concentric circles. Variations in synchronism with the rotation of the spindle motor, commonly referred to as repeatable run-out (RRO), and variations not in synchronism with the rotation, referred to as non-repeatable run-out (NRRO), cause the servo track writer to write tracks which are not concentric circles. For convenience, RRO and NRRO will collectively be referred to as “run-out” in this document. The deviation of the actual written track from the ideal concentric circular track is referred to here as track deviation. Track deviations can be problematic because the write head may end up writing adjacent tracks that overlap. In addition, the read head may not follow the irregularly written track closely enough thereby resulting in what is referred to here as track misregistration. To allow for track deviations, tracks are generally written a certain minimum distance apart, which in turn means that achievable track density is limited.
Zero Acceleration Path (ZAP) techniques may be used to keep the read/write heads on track, and thus minimize track misregistrations. A track is typically divided into sectors, each of which serves as a unit of data storage. ZAP techniques involve writing data patterns in a ZAP field located at the beginning of a sector. The ZAP field and the servo gate contribute to the servo overhead, that is, the portion of the sector that is used for holding servo related information and which is not available for storage of user data. The rest of the sector that can be used for storing user data is referred to in this document as the data field. As the read head travels along a track, the ZAP field is read and calculations are made to estimate the run-out for the whole length of the sector. As technology advances to pack more information onto each disc, it is common to find a single ZAP field being used to estimate the track deviations for an extended stretch of the track. Such attempts to cut down the servo overhead and to free up more space for the data fields may result in compromising the accuracy of the read operations, particularly in cases where the run-out vary significantly along the stretch of the track between consecutive ZAP fields. For example, the ZAP field may be read for a duration of 2.7 microseconds (μs) and yet be used to estimate the run-out information for one or more data fields extending as much as 34.5 μs. If the run-out is more than 29 kilohertz, the resolution loss may well mean that the run-out effects are inadequately compensated for in the positioning of the read head and, as a result, the read head experiences unacceptable track misregistration.
It is expected that the run-out, with the associated problems, will have a greater impact on disc drive performance as the demand for higher format efficiency grows.
There is therefore a need to address the problems described above. The present invention proposes to meet this and other needs, and offers other advantages over the prior art.